The availability of high-gain, high-frequency microwave transistors has revived the old "distributed" or "travelling-wave" approach for broadband microwave amplification. However, instead of electron tubes, these amplifiers employ transistors such as GaAs Field Effect Transistors (FETs). These transistors are ideal active elements for distributed amplifiers in that the intrinsic gate and drain capacitances serve as parts of the shunt elements of two artificial transmission lines: the gate and drain transmission lines. If the line element values (inductances) are chosen properly, wideband amplification can be obtained with more reasonable VSWRs than is possible for an FET having the same total gate width. In order to achieve this performance it is desirable to terminate the gate and drain transmission lines with the characteristic impedance of the lines. However, since the characteristic impedance of such a "constant-k" transmission line is a function of frequency, them is no physical combination of elements that can provide the ideal termination at all frequencies.
Active devices have been used to replace passive resistors in low-noise applications for many years. Recent implementations have made use of GaAs FETs. The transconductance (g.sub.m) of an FET is defined as the ratio of the output (drain-source) signal current to the input (gate) signal voltage. When the drain of an FET is electrically coupled to the gate, the resulting two terminal device appears at signal frequencies as a resistor whose magnitude is equal to 1/g.sub.m. However, more importantly this active termination can achieve an equivalent noise temperature that is significantly colder than is possible with a resistor.